Environment Variable works differently in create-react-app. To access the environment variable in your app, you have to add the prefix
REACT_APP_ before defining your variable as described in the below image.
In our app, you can access this variable as a process.env variable.
let BACKENDURL = process.env.REACT_APP_BACKENDURL || "http://localhost:8080";
You can define the environment variable in the .env file. If you are hosting your app in production like Netlify or Heroku, you can define varibles in their dashboard.